33.5 million people worldwide are infected with HIV and new lines of investigation are required to develop novel anti-viral therapies. In addition to the retroviral gag, pol and env genes, the HIV genome includes six auxiliary genes. Vif is an accessory protein that enhances infectivity of HIV virions, although its specific function remains undefined. It is dispensable for the HIV replication in many T cell lines (permissive lines), but absolutely required for the establishment of infection in a subset of T cell lines (nonpermissive cells) and PBLs, the in vivo targets of HIV infection. Vif expression has also been linked to the cell species restriction of HIV-1 replication. SIVAGM does not replicate in human cells, but co-expression of human Vif restores replicative ability. To delineate protein motifs important for Vif function, chimeric proteins, from a functional HIV-1vif gene and a SIVAGM vif gene will be made. These proteins will be examined in a transcomplementation assay for their ability to complement Vif function in the deltavif HIV- 1 infection of a nonpermissive T cell line. Selective adaptation of an SIVAGM will also be used to identify amino acids important to Vif function. It has also been suggested that Vif functions to overcome an innate anti-viral host cell activity present in nonpermissive cell lines and PBLs. Therefore, the second aim of this proposal is to characterize this novel anti-viral activity using subtractive techniques designed to identify differences between permissive and nonpermissive T cell lines. Identification of host cell factors that mediate Vif function promises to lend insight not only into zoonosis, but also into the interplay between host cell and viral proteins and may lead to the development of novel anti-HIV therapeutics.